We have identified two distinct, mutually nonrecognizing adhesion mechanisms--one Ca++-dependent, the Ca++-independent--on the surfaces of chick embryonic retinal and optic tectal cells. Activity of the CI, but not of the CD, adhesion system corresponds with the dorso-ventral gradient of retinal adhesiveness implicated by others in guiding retinotectal mapping. The possible role of the CD mechanism in a naso-temporal adhesion gradient is currently under investigation. Activity of both retinal adhesion mechanisms is associated with molecules located in the 130-160 k region of Laemmli gels. We are continuing our efforts to prepare monoclonal antibodies against components of each mechanism for use as specific immunological probes in the detailed molecular analysis of these adhesion phenomena. In those analyses, emphasis will be placed initially upon the possible heterogeneity within each adhesion mechanism, upon the nature of the adhesive bonds, and upon the organization of adhesion molecules in the cell membrane. Dual mechanisms functionally compatible with, but physiologically and immunologically distinct from, those of the retina have been described in several other neural (7-day cerebrum, cerebellum and spinal cord) and nonneural (7-day heart and liver, 3 3/4-day limbbud) chick tissues. Efforts will be made to study in detail the structural and functional relationships between these adhesion mechanisms and those of the retina.